It's never mandatory.
But it makes boot and starting a program much faster. Other advantages:
- it uses less power (important for a laptop when it's not connected to the mains)
- there is no need to run a regular defrag on the disk
It doesn't make any difference for small or large calculations.
I am using HDD for a long time, most of my friends are using SSD and insist me to use SSD instead of my HDD. I wonder why is it necessary to switch to SSD. I have no big calculations work on my laptop. Just a casual use laptop
Thanks in advance